An organic light emitting diode (OLED) is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compounds that emit light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes in some cases. Generally, for example, at least one of these electrodes is transparent. OLEDs (based on polymers and/or evaporable small molecules) sometimes are used in television screens; computer monitors; small or portable system screens such as those found on mobile phones and PDAs; and/or the like. OLEDs may also sometimes be used in light sources for space illumination and in large-area light-emitting elements. OLED devices are described in, for example, U.S. Pat. Nos. 7,663,311; 7,663,312; 7,662,663; 7,659,661; 7,629,741; and 7,601,436, the entire contents of each of which are hereby incorporated herein by reference.
A typical OLED comprises two organic layers—namely, electron and hole transport layers—that are embedded between two electrodes. The top electrode typically is a metallic mirror with high reflectivity. The anode is typically a transparent conductive layer supported by a glass substrate. The top electrode generally is the cathode, and the bottom electrode generally is the anode. Indium tin oxide (ITO) often is used for the anode.
FIG. 1 is an example cross-sectional view of an OLED. The glass substrate 102 supports a transparent anode layer 104. The hole transmitting layer 106 may be a carbon nanotube (CNT) based layer in some cases, provided that it is doped with the proper dopants. Conventional electron transporting and emitting and cathode layers 108 and 110 also may be provided.
When a voltage is applied to the electrodes, the charges start moving in the device under the influence of the electric field. Electrons leave the cathode, and holes move from the anode in opposite direction. The recombination of these charges leads to the creation of photons with frequencies given by the energy gap (E=hν) between the LUMO and HOMO levels of the emitting molecules, meaning that the electrical power applied to the electrodes is transformed into light. Different materials and/or dopants may be used to generate different colors, with the colors being combinable to achieve yet additional colors.
The technology has desirable attributes such as large viewing angle, fast response time, high contrast, and a Lambertian profile.
Although significant progress has been made on the electronic quality of the emissive and charge carrier layers, a significant portion of the light emitted is trapped by both the ITO coating on the glass and the underlying glass substrate, e.g., as wave-guiding modes promoted by interference effects. Because of this inefficiency, some of these devices are driven at higher current densities than what normally would be required. This unfortunately has a negative influence on their lifetimes. Even under these non-nominal driving conditions, the luminous efficiency of OLEDs can be significantly below that of fluorescent lamps.
Indeed, it unfortunately is typical for only 20-30% of the photons generated in an OLED on a planar substrate to be extracted into the air. Because of the refractive mismatch between the active OLED layer (where the refractive index is about 1.75 @ 550 nm) and the ITO anode (where the refractive index is about 2.0 @ 550 nm), most of the light is trapped in the device because of total internal reflection, and only a small fraction enters the glass substrate and is actually emitted into the air to serve useful functions.
It would be desirable to improve the light output of an OLED device, e.g., through a light out-coupling strategy. Doing so could improve the lifetime and/or overall luminous efficiency of the device. Several techniques have been proposed to improve the light efficiency, but these methods unfortunately do not meet the practical requirements of manufacturability.
As alluded to above, there have been several attempts to improve techniques for light extraction efficiency. For example, an attempt has been made to increase the extraction from the substrate into the air by way of adding micro-refractive or diffractive structures (e.g., arrays of micro-lenses or pyramids, scattering layers, etc.) to the substrate surface. Depending on the reflectance of the OLED stack, the extraction from the substrate into the air can be increased considerably, typically up to 30%. Unfortunately, however, these structures tend to be quite fragile.
Another attempt relates to monochromatic light emitting devices. In such devices, the angular distribution of the light emitted into the substrate depends on the layer thicknesses of the OLED stack (e.g., by virtue of the micro-cavity effect). By proper design, the amount of light in the escape cone of the substrate can be increased and external efficiencies of up to 40% can be reached at the design wavelength.
Still another approach involves harnessing the “organic modes” that represent about 50% of the generated photons by the introduction of ordered or random scattering structures into the OLED stack. There is a drawback, however, in terms of a possible negative influence on the electrical performance, inasmuch as the anode would be rough, and localized current hot spots that are detrimental to device performance can develop.
A persistent challenge involves attempts at matching the refractive index of the glass substrate and the organic layers so that the organic modes are turned into substrate modes. The amount of light extracted into the substrate can indeed be increased by a factor of 2-3, at least theoretically.
Provided that the OLED has a highly reflective cathode and is thick enough, 80% of the photons generated inside the OLED can be extracted into a high index substrate. However, the remaining issue is still then to out-couple this light into air without reverting back to one of the above-described strategies.
FIG. 2 shows different major light modes in connection with a schematic view of an OLED device. The dashed line in FIG. 2 shows an escape cone. As can be seen, the major modes include a light in air mode (A), which is the fraction of the light that actually emits in the air; a light in substrate mode (B), which is the fraction of the light that is travelling and trapped in the transparent glass substrate; and a light trapped in the organic layers and/or the ITO mode (C), which is the fraction of the light travelling inside and trapped in the organic layers and the high index ITO anode. It will be appreciated that there may be more “B-modes” where the glass is thicker and/or more absorptive. It also is noted that there is another component related to Plasmon losses in the cathode, although this is not depicted in the FIG. 2 schematic view. That is, in a surface Plasmon mode, light is trapped at the organic cathode reflector interface (which oftentimes is an organic to aluminum interface). The failure modes are typical for a bottom emitting OLED device, where light is emitted through the glass substrate.
In view of the foregoing, it will be appreciated that there is a need in the art for techniques for improving the light emitting efficiencies of OLED devices.
One aspect of certain example embodiments relates to a light out-coupling layer stack (OCLS) on a substrate (e.g., on a glass substrate), with a view towards reducing wave-guiding modes.
Another aspect of certain example embodiments relates to scalable techniques for achieving higher luminous efficiency in OLEDs.
Another aspect of certain example embodiments relates to an OCLS structure that includes a vacuum deposited index matching layer (imL) provided over an organo-metallic scattering matrix layer. In certain example embodiments, the imL may be a silicon-inclusive layer and may comprise, for example, silicon oxide, silicon nitride, and/or silicon oxynitride. The imL may be oxygen graded, and thus index graded, in certain example embodiments.
Still another example embodiment relates to an integrated anode glass plate for an OLED or other device. The integrated anode glass plate may include, for example, a soda lime based glass substrate, an OCLS, and an anode comprising ITO or the like. The OCLS may be structured as set forth as noted in the previous paragraph, in certain example instances.
In certain example embodiments, a method of making a coated article is provided. A base scattering matrix layer is wet applied, directly or indirectly, on a glass substrate, e.g., with a precursor for the base scattering matrix layer including an organo-metallic chelate of a high index material and siloxane solvent. The wet applied base scattering matrix layer is cured. A silicon-inclusive index matching layer (e.g., of or including SiOxNy) is vacuum coated, directly or indirectly, on the cured base scattering matrix layer. An anodic layer (e.g., of or including ITO) is vacuum coated, directly or indirectly, on the index matching layer. The cured base scattering matrix layer has a refractive index of 1.55-1.75 (e.g., 1.6-1.7), the index matching layer has a refractive index of 1.7-1.9, and the anodic layer has a refractive index of 1.9-2.1. According to certain example embodiments, the glass substrate may have a refractive index of less than 1.6.
According to certain example embodiments, an additional planarizing layer, a layer comprising GLB may be disposed on the base scattering matrix layer, with the index matching layer being disposed directly over and contacting the layer comprising GLB.
In certain example embodiments, a method of making an electronic device is provided. A coated article (e.g., an integrated anode plate) made according to the example methods herein is provided. The anodic layer is patterned. A hole transport layer, an emitting layer, and a reflective cathodic layer, in that order, are disposed on the patterned anodic layer, in making the electronic device.
The emitting layer may be an electronic transport and emitting layer, and the electronic device may be an OLED-based device, in certain example embodiments. In other example embodiments, the electronic device may be a PLED-based device.
In certain example embodiments, a coated article is provided. The coated article may comprise: a glass substrate; a base scattering matrix layer that is wet applied, directly or indirectly, on the glass substrate, with the base scattering matrix layer including an isotropic layer matrix including an organo-metallic chelate hybrid matrix with scatterers dispersed therein; a silicon-inclusive index matching layer that is sputter-deposited, directly or indirectly, on the base scattering matrix layer; and a transparent conductive coating (TCC) that is sputter-deposited, directly or indirectly, on the index matching layer. The base scattering matrix layer has a refractive index of 1.6-1.7, the index matching layer has a refractive index of 1.7-1.9, and the TCC has a refractive index of 1.9-2.1.
According to certain example embodiments, the cured base scattering matrix layer may be about 3-20 (e.g., more preferably 3-10, and sometimes 5) microns thick and/or may have an average surface roughness (Ra) less than 4 nm. It is noted that the thickness may be increased to a point up until a point where it cracks.
In certain example embodiments, an electronic device is provided. The electronic device may comprise a glass substrate; a base scattering matrix layer that is wet applied, directly or indirectly, on the glass substrate, with the base scattering matrix layer having a thickness of about 3-20 (e.g., 5) microns when cured and including an isotropic layer matrix including an organo-metallic chelate hybrid matrix with high refractive index light scatterers dispersed therein; a silicon-inclusive index matching layer that is sputter-deposited, directly or indirectly, on the base scattering matrix layer; and a first transparent conductive coating (TCC) that is sputter-deposited on the index matching layer. Refractive indices of the glass substrate, the base scattering matrix layer, the index matching layer, and the first TCC may increase with each layer, moving away from the glass substrate. A hole transport layer, an emitting layer, and a reflective second TCC (e.g., a cathode) may be disposed, in that order, over the first TCC.
According to certain example embodiments, the index matching layer may comprise silicon oxynitride and optionally may be at least 200 nm thick, the first TCC may be anodic and optionally may comprise ITC), and the second TCC may be cathodic and may comprise Al, Ag, Pd, Cu, and/or the like, or a combination thereof.
These and other embodiments, features, aspect, and advantages may be combined in any suitable combination or sub-combination to produce yet further embodiments.